Photopolymerizable polymeric compositions used to form relief structures on electrical devices are well known. Also, photosensitive polyamic acid derivatives, which produce thermally stable polyimide films upon cure, have been widely employed for this purpose. Directly patternable photosensitive polyimides simplify relief structure construction in semiconductor manufacture by eliminating conventional photoresists and their attendant processing steps. Negative working photoactive functions can be incorporated into polyimide precursors as polyamic acid esters. These photoactive functions include acrylate, methacrylate, allyl, vinyl, and related double bonds containing structures which can be photopolymerized.
Application of the described photosensitive polyimide precursors or photoresists involves coating a solution of these materials on to a substrate, such as a silicon wafer, and drying to form a film. The film is then exposed to radiation through a masking template and photopolymerized. Unexposed regions, which remain soluble, are removed with a developer solution, leaving behind a relief structure. Thermal curing of the relief structure converts the photopolymerized material into a polyimide coating with pattern definition.
Whereas most photosensitive polyamic acid ester resins are developable after exposure by a range of techniques, including spray, puddle and immersion development, the photosensitive polyamic acid salts most widely employed, process best with polyamic acids which are readily soluble in the common process solvents, such as for example N-methyl-2-pyrrolidone, dimethyacetamide, dimethyl formamide, and dimethyl sulfoxide, and frequently require prolonged immersion or puddle development methods with the commonly employed developers, which usually are composed of various N-methyl-2-pyrolidone, alcohol, water mixtures. Polyamic acid dialkylamine acrylate and methacrylate salts in particular, generally require slow immersion or batch type development for optimum pattern resolution, and therefore are mainly limited to readily soluble polyamic acid systems, in order to be commercially processable. Slow development rates make continuous spray equipment impractical for many polyamic acid salt formulations. Slow development also hinders use of less soluble, rigid polyamic acids, such as that prepared from biphenyl dianhydride and para-phenylene diamine, as their photosensitive dialkylamine acrylate, methacrylate, etc salts. Thus, the desirable features of rigid polyimide structures, such as low thermal expansion, low stress, and excellent mechanical properties, are not readily attainable as a processable photosensitive amine salt precursor. Therefore, it is desirable to prepare photosensitive salts of the rigid, less soluble polyamic acids which can be conveniently processed to well resolved patterned polyimide coatings on different substrates, and especially electronic devices.
Synthesis of the required N-substituted aminoacrylates or methacrylates may be accomplished in by (1) transesterification of aminoalcohols with the corresponding acrylic and methacrylic esters, such as methylacrylate or methylmethacrylate, (2) catalyzed esterification of acrylic and methacrylic acids with aminoalcohols, which requires long reaction intervals, and (3) reaction of aminoalcohols with expensive methacrylic acid chlorides. Acid catalyzed esterification is generally ineffective due to neutralization with the aminoalcohol.
Purification of the ester products has been conducted in the past by distillation under reduced pressure. This procedure, however, subjects the easily polymerizable aminoacrylate, methacrylate, etc., esters to heat for considerable periods during distillation. High boiling products, such as the di- and trimethacrylate esters of aminoalcohols, herein referred to as amino(meth)acrylates, or more generally as amines containing double bonds of the acrylic family, which require greater distillation temperatures, can undergo significant yield loss, due to unwanted polymerization and thermal degradation, even in the presence of selected polymerization inhibitors. Cases where reaction byproducts are removed by extraction and final isolation by solvent removal alone, cannot assure product purity.
Therefore a synthesis and isolation scheme which can produce acrylate and methacrylate esters of mono- and polyhydric aminoalcohols in high yield and purity is also warranted.